Modern industrial cranes are equipped with various sensors, limit switches and protection circuits for safety and efficiency. Faulty operation or incorrect calibration of these sensors causes unnecessary stops or — more dangerously — the crane exceeding its safety limits.
Common Sensor Types and Fault Scenarios
- Limit switch: Stops hoist up/down, bridge forward/back and trolley right/left at their physical limits. Wrong positioning, mechanical breakage or a broken cable causes loss of motion or impact. Calibration: travelling at nominal speed under load, the trip point is set to factory values and the fixing screws torqued to spec.
- Load cell and overload protection: Measures the actual load to trigger overload protection. Wrong calibration either trips early and blocks normal work or trips late and allows overload. Calibration: measured with certified reference weights (10%, 50% and 100% of rated capacity); deviation must be below ±1%.
- Encoder (position / speed sensor): In VFD systems it sends the motor speed and position. Cable damage, wrong pulse count or mechanical slip cause error codes (e.g. F0011, E.PID). Calibration: the pulse/rev value in the VFD parameters is matched to the motor nameplate; cable shielding and earthing are checked.
- Distance / anti-collision sensor: Maintains the minimum safe distance between cranes sharing a runway. A dirty sensor lens or wrong threshold causes false stops or collisions. Calibration: the sensor is cleaned and the minimum safe distance (typically 1.5–2 m) verified on site with test loads.
Sensor Maintenance and Calibration Schedule
- Monthly: Physical condition and function test of the limit switches.
- Quarterly: Load cell zero-offset check.
- Annually: Full calibration certification (TSE requirements).
- Post-incident: All sensors must be checked after any impact or overload event.
Frequently Asked Questions
What is the acceptable deviation in load cell calibration?
In measurements with certified reference weights the deviation must be below ±1%. Verification is done with systematic increments at 10%, 50% and 100% of rated capacity.
Why is a limit switch fault dangerous?
A limit switch stops motion at its physical limit. When it fails, the crane either does not move in that direction or travels without stopping and strikes the buffer/structure. The hoist-up limit switch is especially safety-critical.